The present invention relates to a melt-processable fluorine-containing resin composition having an excellent thermal stability, and more particularly to the composition comprising a tetrafluoroethylene (hereinafter referred to as "TFE") copolymer, a chlorotrifluoroethylene (hereinafter referred to as "CTFE") homo- or co-polymer or a vinylidene fluoride (hereinafter referred to as "VdF") homo- or co-polymer, and a thermal stabilizer, which has an improved thermal stability to high-temperature sintering and is processable under widen processing conditions with technical and economical advantages and also can provide an article having excellent physical properties.
TFE copolymers and CTFE or VdF homopolymer and copolymers are melt-processable fluorine-containing resins having especially high thermal resistance among those put on the market, and for instance, as the former there are known TFE-hexafluoropropylene copolymer, perfluorovinyl ether-TFE copolymer, ethylene-TFE copolymer and ethylene-propylene-TFE copolymer and as the latter there are known CTFE and VdF homopolymers, ethylene-CTFE copolymer and VdF-TFE copolymer, etc. These melt-processable fluorine-containing resins have melt-flowability, that is, the melt viscosity of these resins is generally lower than 10.sup.6 poises at an optimum processing temperature and, therefore, they provide a film having less pinholes and voids as compared with polytetrafluoroethylene which has an excellent chemical and corrosion resistance but has not melt-flowability or melt-processability, since it has an extremely high melt viscosity of from 10.sup.10 to 10.sup.11 poises even at a processing temperature, i.e. about 380.degree. C.
The thermal stability of these melt-processable resins at high temperatures in the vicinity of their sintering temperatures is, however, inferior to that of polytetrafluoroethylene, and this makes some troubles in processing. That is to say, when the resins are heated at a suitable sintering temperature of 340.degree. to 380.degree. C. for a time more than 30 minutes, the resins partially cause thermal deterioration, and particularly when the coated film is considerably thick, bubbles are formed in the film inevitably. This phenomenon is accelerated by the influence of oxygen in air.
For this reason, in case of TFE-hexafluoropropylene resin, for instance, there are proposed (1) a process in which thickness of the coating per one application is controlled as small as possible (about 50.mu.) and the application and sintering procedures must be repeated many times until a sintered film reaches a desired thickness, and (2) a process in which a resin having a low molecular weight (of which melt viscosity is about 0.5.times.10.sup.4 to about 7.times.10.sup.4 poises at 380.degree. C.) or a resin obtained by heat treatment of a high molecular weight resin (the melt viscosity of the high molecular weight resin is from about 1.times.10.sup.5 to about 4.times.10.sup.5 poises at 380.degree. C.) is used for a coating composition so that the resin melts and flows at a lower temperature, and the coating is sintered at a lower temperature (320.degree. to 340.degree. C.) to give a sintered film.
However, the above process (1) has the disadvantage that the formation of a film having a thickness necessary in general for corrosion resistant linings, i.e. about 600 to about 1,000.mu. requires much labor and time in application and sintering.
Also, the above process (2) accompanies the formation of bubbles in a coated film when the thickness of the film per one application exceeds 100.mu., even though the sintering has been conducted at a lower temperature (320.degree. to 340.degree. C.) Therefore, when it is desired to obtain a film having a thickness of more than 1,000.mu., the application and sintering must be repeated more than 10 times as in the process (1). Thus, the process (2) is also low in productivity and is not economical. Further, a low molecular weight resin is inferior in stress crack resistance and solvent crack resistance and is not desirable as a corrosion resistant material. Moreover, thermal resistance of such a resin is low, the allowable range for processing temperature and period of time are narrow, and the thermal deterioration may take place during the processing. And further such a low molecular weight resin is liable to occur runs during the processing. When a lining is made on an industrial scale for a large-sized substrate, for instance, having a length of more than one meter or a substrate having an irregular thickness, temperature distribution on the surface of the substrate and difference in heat history become, of course, large, and in such a case a uniform lining of good quality is hard to obtain by the process (2).
Also, in case of ethylene-propylene-TFE copolymer and ethylene-CTFE copolymer, the bubble formation upon sintering after powder coating is not so much as TFE-hexafluoropropylene copolymer. However, when the sintering for a long time is required owing to the size and shape of a substrate to be coated, it accompanies deterioration of the resin, and as a result, the obtained film is discolored and also the durability to various environments and chemical reagents is remarkably impaired.
There are known various processes for improving the thermal stability of these melt-processable fluorine-containing resins upon sintering. For instance, Japanese Unexamined Patent Publication Nos. 122155/1976 and 122156/1976 disclose processes for improving the thermal stability of the resins by admixing two kinds of TFE-hexafluoropropylene copolymer with different melt viscosities which are thermally treated at a high temperature in the presence of steam. These processes require not only the thermal treatment of TFE-hexafluoropropylene copolymer at a high temperature of 340.degree. to 380.degree. C. for 2 to 5 hours, but also drying for several hours to remove moisture because of the thermal treatment in the presence of steam, and accordingly is not economical.
It is also known to use, as a thermal stabilizer for ethylene-TFE copolymer, sulfates of metals of Group IV-A of the Periodic Table such as Sn and Pb as disclosed in Japanese Patent Publication No. 37980/1973; phosphates of alkali metals, barium or metals of Group IV-A of the Periodic Table as disclosed in Japanese Patent Publication No. 37981/1973; a combination of organo phosphites and phosphates of alkali metals, barium or metals of Group IV-A of the Periodic Table as disclosed in Japanese Patent Publication No. 38215/1973; and .alpha.-alumina as disclosed in Japanese Unexamined Patent Publication No. 87738/1974. However, these thermal stabilizers merely inhibit the discoloration by thermal degradation of ethylene-TFE copolymer in the sintering at 300.degree. C. within 30 minutes, and are not suitable for use in coating a substrate to be coated having a large size and a large heat capacity.